This invention relates to a device for measuring the recirulating flow rate of the primary coolant (reactor coolant) in a boiling water reactor (BWR) in which an internal pump is installed on the bottom of a reactor pressure vessel (called hereinafter an internal pump type reactor), particularly for measuring the flow rate of the primary coolant precisely and easily.
In one typical example of a known device for measuring primary coolant recirculating flow rate of a BWR, differential pressure detectors are installed on the upstream and downstream sides of a core support plate which supports a fuel assembly in the reactor, and the pressure difference caused by the core support plate is detected by a pressure difference transmitter as a pressure difference at the core support plate thereby to measure the recirculating flow rate (core flow rate). In the recirculating flow rate measuring device of this prior type, the core support plate constitutes a flow path resistance, and the flow rate is obtained or calculated from the pressure difference between the upstream side and the downstream side of the core support plate.
With a device of the type described above, however, since the pressure difference at the core support plate is measured as a pressure including a pressure difference in the interior of the fuel assembly, when two phase flow conditions of the liquid phase and the gas phase in the fuel assembly vary in accordance with the reactor output, the resisting value also varies, and accordingly, the flow rate cannot be exactly measured.
In another recirculating flow rate measuring device which has been proposed in this field, differential pressure detectors are located on the upstream and downstream sides of an internal pump to detect the pressure difference caused by the location of the internal pump by a pressure difference transmitter as a pressure difference at the internal pump, and in which the rotational speed of the internal pump is detected by a pulse converter. The recirculating flow rate (pump flow rate) is then measured by the combination of the pressure difference at the internal pump and the rotational speed thereof.
With the flow rate measuring device of the latter mentioned type, however, it is generally necessary to preliminarily determine the relationship between the pressure difference at the internal pump and the rotational speed thereof and the flow rate, but this relationship is often different with the internal pump used. Moreover, even with the same internal pump, the relationship for determining the flow rate may be different depending on the operational conditions thereof. Furthermore, with a device of this character, since the pressure difference at the internal pump and the rotational speed thereof are taken as factors to be inputted, the construction of the device is made complicated, and the accurate measuring of the flow rate cannot be attained.
As still another recirculating flow rate device, the prior art provides another type in which a flow meter of general type such as a Venturi tube is installed in the reactor (for example, refer to Japanese Patent Laid-Open Publication No. 144594/1980), but a device of this type involves drawbacks of complicated structure and increasing of the flow resistance of the primary coolant.